Chambers for processing subjects under vacuum are usually manufactured from stainless steel or aluminum in that individual plates of steel or plates of aluminum are welded. For welding individual plates, said plates are clamped into a device and welded either manually or by a robotic device.
It is difficult to weld the plates of steel or plates of aluminum on the inside of a chamber, in particular in case of large scale chambers as used for coating architectural glass or solar cells by vacuum processes, and/or in case of welding by a robotic device. Therefore, the plates of such vacuum processing chambers are often welded on their outside only. As the plates for manufacturing a vacuum processing chamber may have a thickness of several centimeters, depending on the size of the chamber, gaps remain between the individual plates on the inside of the chamber.
These gaps are virtual leaks and may cause an adverse performance of the vacuum processing chamber. Particularly the pumping-off characteristics of a chamber manufactured this way is bad, because processing additives such as cooling media or oil enter the gaps and affect fumigation characteristics. In addition, said additives may interfere with the deposition process and impair the quality of the substrate.
To avoid such disturbing gaps, vacuum process chambers are manufactured from stainless steel, because plates of stainless steel can be welded easier on the inside of a chamber to be manufactured than aluminum. However, a process module or a process chamber made of stainless steel may possess problems if corrosive media are used. In current coating processes as used in industrial manufacturing of solar cells or other semi conductor products, the process chambers are regularly cleaned in situ. For this purpose, very often cleaning gases are utilized which are activated in a remote plasma source (RPS) or—in case of PECVD facilities—by the parallel electrodes of the reactor. The radicals generated thereby etch off the reaction products of previous coating processes and are subsequently removed from the process chamber by the vacuum pumps that are fitted to the chamber after suitable treatment of the etching products. To save time, corrosive chloride compounds and fluoride compounds (SF6, NF3) are employed. Upon use of such cleaning gases, the resistance of modern, commercially available steels to corrosion is insufficient, such that corrosion phenomena may occur in long term.
For large scale vacuum processing chambers, reinforcing ribs have to be provided to the side walls of the chamber such that the thickness of the walls may be reduced, but stiffness of the walls remains sufficient. However, providing vacuum processing chambers with reinforcing ribs requires tremendous manufacturing efforts and increases costs.
An alternative to welded vacuum processing chambers are vacuum processing chambers that were manufactured by casting, particularly vacuum processing chambers made from aluminum by means of sand casting. Manufacturing vacuum processing chambers by casting provides advantages, in particular if large scale vacuum processing chambers can be made by a single casting.
Briefly, the advantages are:
I. Many more opportunities in designing the chamber to be casted than a welded construction would allow. The opportunity to freely choose the number and positions of reinforcing rips permits to purposely use material where it is really needed at the chamber. For example, an area with high load may be provided with larger or stronger rips that a less loaded area. A slim production of the chamber enabled thereby reduces costs for material and transport.
II. Casting processes permit avoidance of advert dead spaces and gaps in a chamber to be manufactured.
III. Smooth transitions and/or transitions with any radius between individual walls of the chamber can be accomplished.
IV. The use of aluminum instead of stainless steel reduces the weight or the chamber. In addition, aluminum is known to be more resistant to those cleaning gases that are used today and will be used in future.
V. The freedom in designing the chamber permits reducing the chamber volume, because the walls of the chamber may be adapted to the components to processed within the chamber.
VI. A casting form made of wood or plastics for sand casting can be modified easily. This reduces the efforts to be made in case of subsequent chamber modifications.
A general problem in casting, in particular in sand casting, are the unavoidable pores in the surface of the casted part. Utilizing special cooling elements at the casting mould may substantially prevent pore forming at surfaces to be treated later, in particular at sealing surfaces. However, this method is too complicated and elaborate for extensive use. Thus, the untreated surfaces inside the casted chamber have to remain relatively rough. To improve the quality of these surfaces, they are smoothed by abrasion and crude polishing.
By said smoothening, the inner surface of the chamber is generally reduced. This is an important aspect, in particular for vacuum processing chambers, because the inner surface of the vacuum processing chamber constitutes an adsorption area whereat deposition vapours, oxygen, water, nitrogen and the like may be adsorbed.
On the other hand, said abrasion leads to the formation of microscopic cavities by the aluminum flow. Small recesses in the casted material may be plugged with particles that are produced during abrasion. The essential step of improving the inner surface of the vacuum processing chamber also causes fixation of abrasives, material obtained by the abrasion, residual cooling agent and the like in the cavities and recesses of the inner face of the vacuum processing chamber. However, these materials tend to gas-out during evacuation of the vacuum processing chamber—the desired pressure for processing is reached later.
Additionally, the compounds that gas-out may affect the subsequent vacuum processes. The usual and prescribed cleaning processes for a vacuum processing chamber do not or insufficiently reach those particles within the pores, cavities or recesses.
For these reasons, vacuum processing chambers made of aluminum by casting are rarely used nowadays.